This invention relates to improvements in a fuel vapor storage canister as a fuel vapor treatment device for treating fuel vapor generated, for example, from a fuel tank of an automotive vehicle.
Hitherto a fuel vapor storage canister as a fuel vapor treatment device has been generally used in order to suppress release of fuel vapor generated within a fuel tank to atmospheric air in a gasoline-fueled automotive vehicle, as disclosed in Japanese Patent Provisional Publication No. 2005-23835. This canister includes a casing formed thereinside a gas passage which is filled with a fuel vapor adsorbent such as activated carbon. Charge and purge ports for fuel vapor are provided at one end of the gas passage, while a drain port for fuel vapor is provided at the other end of the gas passage. During stopping of the vehicle, fuel vapor generated from the fuel tank is introduced into the canister and adsorbed by the adsorbent. During operation of an engine, atmospheric air is introduced through the drain port so as to purge fuel vapor adsorbed in the adsorbent and supply the purged fuel vapor to an intake system of the engine through the purge port so that the supplied fuel vapor is combusted and treated within the engine, thus accomplishing a so-called purging. With such purging, a fuel vapor adsorbing performance of the canister can be revived thereby allowing the adsorbent to repeatedly adsorb fuel vapor.
In recent years, environmental protection regulations have become more strict, and therefore a performance improvement is desired for the canister. In order to attain the performance improvement of the canister, it is effective to improve a purging efficiency for fuel vapor during the above purging. Regarding the purging efficiency, important factors for lowering the purging efficiency are a temperature lowering of activated carbon during the purging, and non-uniformization of flow of purge gas or air throughout the cross-sectional area of the gas passage. Accordingly, in order to improve the purging efficiency, it is effective to solve or improve the important factors.
Additionally, other measures for improving the purging efficiency include enlarging a ratio L/D (the length L of the gas passage/the diameter D of the gas passage), and minimizing the volume of a part of the gas passage which part is near the drain port, thereby increasing the amount of the purge air per unit volume. With this, fuel vapor which cannot be trapped at an upstream part on the charge port side can be effectively trapped by a downstream part on the drain port side during charging of fuel vapor.
In order to accomplish such improvements in purging efficiency, in the canister as disclosed in the above Japanese Patent Provisional Publication, the gas passage is arranged including two longitudinal direction passages formed along a casing longitudinal direction, and a U-turn passage for connecting the ends of both the longitudinal direction passages in order to ensure a sufficient length of the gas passage within a compact casing. Additionally, the longitudinal direction passage on the side of the charge and purge ports is formed with one activated carbon layer (adsorption layer) including activated carbon, while the longitudinal direction passage on the side of the drain port or downstream side is formed with two activated carbon layers between which a spacer (or diffusion-retarding cartridge) is interposed thereby reducing the volume of the adsorption layer near the drain port so as to increase a purge air amount per unit volume.